A substrate-specific mTORC1 pathway underlies Birt-Hogg-Dubé syndrome.

The mechanistic target of rapamycin complex 1 (mTORC1) is a key metabolic hub that controls the cellular response to environmental cues by exerting its kinase activity on multiple substrates1-3. However, whether mTORC1 responds to diverse stimuli by differentially phosphorylating specific substrates is poorly understood. Here we show that transcription factor EB (TFEB), a master regulator of lysosomal biogenesis and autophagy4,5, is phosphorylated by mTORC1 via a substrate-specific mechanism that is mediated by Rag GTPases. Owing to this mechanism, the phosphorylation of TFEB-unlike other substrates of mTORC1, such as S6K and 4E-BP1- is strictly dependent on the amino-acid-mediated activation of RagC and RagD GTPases, but is insensitive to RHEB activity induced by growth factors. This mechanism has a crucial role in Birt-Hogg-Dubé syndrome, a disorder that is caused by mutations in the RagC and RagD activator folliculin (FLCN) and is characterized by benign skin tumours, lung and kidney cysts and renal cell carcinoma6,7. We found that constitutive activation of TFEB is the main driver of the kidney abnormalities and mTORC1 hyperactivity in a mouse model of Birt-Hogg-Dubé syndrome. Accordingly, depletion of TFEB in kidneys of these mice fully rescued the disease phenotype and associated lethality, and normalized mTORC1 activity. Our findings identify a mechanism that enables differential phosphorylation of mTORC1 substrates, the dysregulation of which leads to kidney cysts and cancer.

PRDM10 directs FLCN expression in a novel disorder overlapping with Birt-Hogg-Dubé syndrome and familial lipomatosis.

Birt-Hogg-Dubé syndrome (BHD) is an autosomal dominant disorder characterized by fibrofolliculomas, pulmonary cysts, pneumothoraces and renal cell carcinomas. Here, we reveal a novel hereditary disorder in a family with skin and mucosal lesions, extensive lipomatosis and renal cell carcinomas. The proband was initially diagnosed with BHD based on the presence of fibrofolliculomas, but no pathogenic germline variant was detected in FLCN, the gene associated with BHD. By whole exome sequencing we identified a heterozygous missense variant (p.(Cys677Tyr)) in a zinc-finger encoding domain of the PRDM10 gene which co-segregated with the phenotype in the family. We show that PRDM10Cys677Tyr loses affinity for a regulatory binding motif in the FLCN promoter, abrogating cellular FLCN mRNA and protein levels. Overexpressing inducible PRDM10Cys677Tyr in renal epithelial cells altered the transcription of multiple genes, showing overlap but also differences with the effects of knocking out FLCN. We propose that PRDM10 controls an extensive gene program and acts as a critical regulator of FLCN gene transcription in human cells. The germline variant PRDM10Cys677Tyr curtails cellular folliculin expression and underlies a distinguishable syndrome characterized by extensive lipomatosis, fibrofolliculomas and renal cell carcinomas.

Update of penetrance estimates in Birt-Hogg-Dubé syndrome.

BACKGROUND: Birt-Hogg-Dubé (BHD) syndrome is a rare genetic syndrome caused by pathogenic or likely pathogenic germline variants in the FLCN gene. Patients with BHD syndrome have an increased risk of fibrofolliculomas, pulmonary cysts, pneumothorax and renal cell carcinoma. There is debate regarding whether colonic polyps should be added to the criteria. Previous risk estimates have mostly been based on small clinical case series. METHODS: A comprehensive review was conducted to identify studies that had recruited families carrying pathogenic or likely pathogenic variants in FLCN. Pedigree data were requested from these studies and pooled. Segregation analysis was used to estimate the cumulative risk of each manifestation for carriers of FLCN pathogenic variants. RESULTS: Our final dataset contained 204 families that were informative for at least one manifestation of BHD (67 families informative for skin manifestations, 63 for lung, 88 for renal carcinoma and 29 for polyps). By age 70 years, male carriers of the FLCN variant have an estimated 19% (95% CI 12% to 31%) risk of renal tumours, 87% (95% CI 80% to 92%) of lung involvement and 87% (95% CI 78% to 93%) of skin lesions, while female carriers had an estimated 21% (95% CI 13% to 32%) risk of renal tumours, 82% (95% CI 73% to 88%) of lung involvement and 78% (95% CI 67% to 85%) of skin lesions. The cumulative risk of colonic polyps by age 70 years old was 21% (95% CI 8% to 45%) for male carriers and 32% (95% CI 16% to 53%) for female carriers. CONCLUSIONS: These updated penetrance estimates, based on a large number of families, are important for the genetic counselling and clinical management of BHD syndrome.

Clinical phenotype and genetic function analysis of a rare family with hereditary leiomyomatosis and renal cell carcinoma complicated with Birt-Hogg-Dubé syndrome.

To date, over 200 families with hereditary leiomyomatosis and renal cell carcinoma (HLRCC) and over 600 families with Birt-Hogg-Dubé (BHD) syndrome have been reported, with low incidence. Here, we describe a patient with suspected rare HLRCC complicated by BHD syndrome. The proband (II1) had characteristic cutaneous leiomyoma-like protrusions on the neck and back, a left renal mass and multiple right renal, liver and bilateral lung cysts. Three family members (I1, II2, II3) had a history of renal cancer and several of the aforementioned clinical features. Two family members (II1, II3) diagnosed with fumarate hydratase (FH)-deficient papillary RCC via pathological biopsy carried two heterozygous variants: FH (NM_000143.3) missense mutation c.1189G>A (p.Gly397Arg) and FLCN (NM_144997.5) frameshift mutation c.1579_1580insA (p.Arg527Glnfs*75). No family member carrying a single variant had renal tumours. In HEK293T cells transfected with mutant vectors, mRNA and protein expression after FLCN p.Arg527Glnfs*75 and FH p.Gly397Arg mutations were significantly lower than those in wild-type (WT) cells. Cell immunofluorescence showed altered protein localisation and reduced protein expression after FLCN p.Arg527Glnfs*75 mutation. The FH WT was uniformly distributed in the cytoplasm, whereas FH protein expression was reduced after the p.Gly397Arg mutation and scattered sporadically with altered cell localisation. Patients with two variants may have a significantly increased penetrance of RCC.

Phosphoproteomic Analysis of FLCN Inactivation Highlights Differential Kinase Pathways and Regulatory TFEB Phosphoserines.

In Birt-Hogg-Dubé (BHD) syndrome, germline loss-of-function mutations in the Folliculin (FLCN) gene lead to an increased risk of renal cancer. To address how FLCN inactivation affects cellular kinase signaling pathways, we analyzed comprehensive phosphoproteomic profiles of FLCNPOS and FLCNNEG human renal tubular epithelial cells (RPTEC/TERT1). In total, 15,744 phosphorylated peptides were identified from 4329 phosphorylated proteins. INKA analysis revealed that FLCN loss alters the activity of numerous kinases, including tyrosine kinases EGFR, MET, and the Ephrin receptor subfamily (EPHA2 and EPHB1), as well their downstream targets MAPK1/3. Validation experiments in the BHD renal tumor cell line UOK257 confirmed that FLCN loss contributes to enhanced MAPK1/3 and downstream RPS6K1/3 signaling. The clinically available MAPK inhibitor Ulixertinib showed enhanced toxicity in FLCNNEG cells. Interestingly, FLCN inactivation induced the phosphorylation of PIK3CD (Tyr524) without altering the phosphorylation of canonical Akt1/Akt2/mTOR/EIF4EBP1 phosphosites. Also, we identified that FLCN inactivation resulted in dephosphorylation of TFEB Ser109, Ser114, and Ser122, which may be linked to increased oxidative stress levels in FLCNNEG cells. Together, our study highlights differential phosphorylation of specific kinases and substrates in FLCNNEG renal cells. This provides insight into BHD-associated renal tumorigenesis and may point to several novel candidates for targeted therapies.

Kidney cancer: Links between hereditary syndromes and sporadic tumorigenesis.

Multiple hereditary syndromes predispose to kidney cancer, including Von Hippel-Lindau syndrome, BAP1-Tumor Predisposition Syndrome, Hereditary Papillary Renal Cell Carcinoma, Tuberous Sclerosis Complex, Birt-Hogg-Dubé syndrome, Hereditary Paraganglioma-Pheochromocytoma Syndrome, Fumarate Hydratase Tumor Predisposition Syndrome, and Cowden syndrome. In some cases, mutations in the genes that cause hereditary kidney cancer are tightly linked to similar histologic features in sporadic RCC. For example, clear cell RCC occurs in the hereditary syndrome VHL, and sporadic ccRCC usually has inactivation of the VHL gene. In contrast, mutations in FLCN, the causative gene for Birt-Hogg-Dube syndrome, are rarely found in sporadic RCC. Here, we focus on the genes and pathways that link hereditary and sporadic RCC.

Birt-Hogg-Dubé syndrome in an overall view: Focus on the clinicopathological prospects in renal tumors.

Birt-Hogg-Dubé syndrome (BHD) represents a rare autosomal dominant tumor predisposition syndrome characterized by skin lesions, lung cysts, and renal tumors. The predominant histological subtypes of BHD-related renal tumors include hybrid oncocytoma-chromophobe tumors, oncocytomas, and chromophobe renal cell carcinomas, all exhibiting eosinophilic/oncocytic features. Immunohistochemistry staining for KIT (CD117) and CK7 exhibits variability in these tumor types. Germline mutations in FLCN have been consistently identified. Generally, patients with BHD demonstrate a favorable prognosis with minimal metastatic potential. Nonetheless, the comprehensive elucidation of pathological characteristics of BHD remains incomplete, particularly in BHD-associated renal tumors that deviate from the previously identified subtypes, thereby complicating the differential diagnosis. In this review, we provide a comprehensive overview of BHD encompassing epidemiology, clinical manifestations, genetic and molecular pathogenesis, as well as clinical diagnostic modalities. Emphasis is placed on clinicopathological features, specifically focusing on BHD-associated renal tumors. Collectively, this review aims to present the latest insights into BHD which benefits in the early detection, therapeutic decision-making, and prognosis prediction in BHD cases, and deepen the understanding of sporadic renal tumors.

[Diffuse cystic lung disease]. / Cystische Lungenerkrankungen.

INTRODUCTION: Diffuse cystic lung disease (DCLD) represents a heterogeneous group of conditions, typically characterized by the presence of multiple thin-walled, predominantly round parenchymal lucencies. The increased accessibility of computed tomography (CT) underscores the growing relevance of a relatively rare group of diseases as more clinicians are confronted with the presence of multiple lung cysts on the chest CT scan. Although the etiology of these conditions is very diverse, the focus of the differential diagnosis revolves around four primary causative factors - Lymphangioleiomyomatosis (LAM), Pulmonary Langerhanscell histiocytosis (PLCH), Birt-Hogg-Dubé (BHD) and lymphoid interstitial pneumonia (LIP). Achieving an accurate diagnosis poses a challenge and typically necessitates lung biopsies; however, it is crucial for ensuring proper management.

Familial multiple discoid fibromas is linked to a locus on chromosome 5 including the FNIP1 gene.

Previously, we reported a series of families presenting with trichodiscomas, inherited in an autosomal dominant pattern. The phenotype was named familial multiple discoid fibromas (FMDF). The genetic cause of FMDF remained unknown so far. Trichodiscomas are skin lesions previously reported to be part of the same spectrum as the fibrofolliculoma observed in Birt-Hogg-Dubé syndrome (BHD), an inherited disease caused by pathogenic variants in the FLCN gene. Given the clinical and histological differences with BHD and the exclusion of linkage with the FLCN locus, the phenotype was concluded to be distinct from BHD. We performed extensive clinical evaluations and genetic testing in ten families with FMDF. We identified a FNIP1 frameshift variant in nine families and genealogical studies showed common ancestry for eight families. Using whole exome sequencing, we identified six additional rare variants in the haplotype surrounding FNIP1, including a missense variant in the PDGFRB gene that was found to be present in all tested patients with FMDF. Genome-wide linkage analysis showed that the locus on chromosome 5 including FNIP1 was the only region reaching the maximal possible LOD score. We concluded that FMDF is linked to a haplotype on chromosome 5. Additional evaluations in families with FMDF are required to unravel the exact genetic cause underlying the phenotype. When evaluating patients with multiple trichodisomas without a pathogenic variant in the FLCN gene, further genetic testing is warranted and can include analysis of the haplotype on chromosome 5.

A diagnosis of Birt-Hogg-Dubé syndrome in individuals with Smith-Magenis syndrome: Recommendation for cancer screening.

We report a series of four unrelated adults with Smith-Magenis syndrome (SMS) and concomitant features of Birt-Hogg-Dubé (BHD) syndrome based upon haploinsufficiency for FLCN and characteristic renal cell carcinomas and/or evidence of cutaneous fibrofolliculomas. Three of the cases constitute the first known association of histopathologically verified characteristic BHD-associated renal tumors in adults with SMS; the fourth was identified to have histologically confirmed skin fibrofolliculomas. Molecular analysis documented second-hit FLCN mutations in two of the three cases with confirmed BHD renal pathology. These cases suggest the need to expand management recommendations for SMS to include kidney cancer surveillance starting at 20 years of age, as per the screening recommendations for BHD syndrome.

Pulmonary interstitial glycogenosis in Birt-Hogg-Dubé syndrome-associated lung cysts: A new insight into the pathogenesis?

Multiple lung cysts are one of the major features of Birt-Hogg-Dubé syndrome (BHD), but little is known about their nature and pathogenesis. We report a case of a woman diagnosed with BHD lung cysts who exhibited pulmonary interstitial glycogenosis (PIG), a mesenchymal abnormality hitherto undescribed in this disease, in specimens resected at 14 and 29 years of age. Histopathologically, oval to spindle clear cells were seen in the subepithelial interstitial tissue of septal structures and the walls of the cysts. They had abundant periodic acid-Schiff-positive cytoplasmic glycogen. Immunohistochemically, these cells were positive for a few markers of mesenchymal stem cell-like lineage, including vimentin, CD44, and CD10, and negative for markers of epithelial or specific mesenchymal differentiation; these results were consistent with the reported immunophenotype of PIG cells. These PIG cells were more abundant in her specimen at age 14 years than in the second specimen from adulthood. The present case suggests that BHD lung cysts belong to a group of pulmonary developmental disorders characterized by combined PIG and alveolar simplification/cystic change. Disorders with PIG may persist until adulthood and may be of clinical and pathological significance.

Coexistent Sjogren's syndrome and Birt-Hogg-Dube´ syndrome: a case report.

We report a rare case of Sjogren's syndrome complicated with Birt-Hogg-Dubé syndrome (BHDS) not previously mentioned in the literature. Further, there is insufficient evidence linking the two diseases. Here, we review existing diagnostic algorithms for diagnosing diffuse cystic lung disease and provide new insights. The patient initially complained of thirst and dry eyes for ten years, and gradually developed shortness of breath. After admission, physical examination showed five missing teeth, decreased respiratory sounds in both lower lungs, and Velcro rales. Computed tomography showed multiple thin-walled cystic lesions in both lungs. Initial xerophthalmia and labial gland biopsy seemed to reveal a pulmonary cystic change associated with Sjogren's syndrome. Before discharge, a rash suspected to indicate a fibrofollicular tumor in the neck was observed, and then FLCN variant has been found. The challenges how to clarify the diagnosis of DCLD causes are discussed.

Progression of pulmonary cysts in Birt-Hogg-Dubé syndrome: longitudinal thoracic computed tomography study with quantitative assessment.

BACKGROUND: Birt-Hogg-Dubé (BHD) syndrome is a rare autosomal dominant disorder characterized by fibrofolliculomas, renal tumors, pulmonary cysts, and recurrent pneumothorax. Pulmonary cysts are the cause of recurrent pneumothorax, which is one of the most important factors influencing patient quality of life. It is unknown whether pulmonary cysts progress with time or influence pulmonary function in patients with BHD syndrome. This study investigated whether pulmonary cysts progress during long-term follow-up (FU) by using thoracic computed tomography (CT) and whether pulmonary function declines during FU. We also evaluated risk factors for pneumothorax in patients with BHD during FU. METHODS: Our retrospective cohort included 43 patients with BHD (25 women; mean age, 54.2 ± 11.7 years). We evaluated whether cysts progress by visual assessment and quantitative volume analysis using initial and serial thoracic CT. The visual assessment included the size, location, number, shape, distribution, presence of a visible wall, fissural or subpleural cysts, and air-cuff signs. In CT data obtained from a 1-mm section from 17 patients, the quantitative assessment was performed by measuring the volume of the low attenuation area using in-house software. We evaluated whether the pulmonary function declined with time on serial pulmonary function tests (PFT). Risk factors for pneumothorax were analyzed using multiple regression analysis. RESULTS: On visual assessment, the largest cyst in the right lung showed a significant interval increase in size (1.0 mm/year, p = 0.0015; 95% confidence interval [CI], 0.42-1.64) between the initial and final CT, and the largest cyst in the left lung also showed significant interval increase in size (0.8 mm/year, p < 0.001, 95% CI; -0.49-1.09). On quantitative assessment, cysts had a tendency to gradually increase in size. In 33 patients with available PFT data, FEV1pred%, FEV1/FVC, and VCpred% showed a statistically significant decrease with time (p < 0.0001 for each). A family history of pneumothorax was a risk factor for the development of pneumothorax. CONCLUSIONS: The size of pulmonary cysts progressed over time in longitudinal follow-up thoracic CT in patients with BHD, and pulmonary function had slightly deteriorated by longitudinal follow-up PFT.

Folliculin variants linked to Birt-Hogg-Dubé syndrome are targeted for proteasomal degradation.

Germline mutations in the folliculin (FLCN) tumor suppressor gene are linked to Birt-Hogg-Dubé (BHD) syndrome, a dominantly inherited genetic disease characterized by predisposition to fibrofolliculomas, lung cysts, and renal cancer. Most BHD-linked FLCN variants include large deletions and splice site aberrations predicted to cause loss of function. The mechanisms by which missense variants and short in-frame deletions in FLCN trigger disease are unknown. Here, we present an integrated computational and experimental study that reveals that the majority of such disease-causing FLCN variants cause loss of function due to proteasomal degradation of the encoded FLCN protein, rather than directly ablating FLCN function. Accordingly, several different single-site FLCN variants are present at strongly reduced levels in cells. In line with our finding that FLCN variants are protein quality control targets, several are also highly insoluble and fail to associate with the FLCN-binding partners FNIP1 and FNIP2. The lack of FLCN binding leads to rapid proteasomal degradation of FNIP1 and FNIP2. Half of the tested FLCN variants are mislocalized in cells, and one variant (ΔE510) forms perinuclear protein aggregates. A yeast-based stability screen revealed that the deubiquitylating enzyme Ubp15/USP7 and molecular chaperones regulate the turnover of the FLCN variants. Lowering the temperature led to a stabilization of two FLCN missense proteins, and for one (R362C), function was re-established at low temperature. In conclusion, we propose that most BHD-linked FLCN missense variants and small in-frame deletions operate by causing misfolding and degradation of the FLCN protein, and that stabilization and resulting restoration of function may hold therapeutic potential of certain disease-linked variants. Our computational saturation scan encompassing both missense variants and single site deletions in FLCN may allow classification of rare FLCN variants of uncertain clinical significance.

A Novel FLCN Variant in a Suspected Birt-Hogg-Dubè Syndrome Patient.

Subjects with pathogenic (PV) and likely pathogenic (LPV) FLCN variants have an increased risk of manifesting benign and malignant disorders that are related to Birt-Hogg-Dubé syndrome (BHDS): an autosomal dominantly inherited disorder whose severity can vary significantly. Renal cell carcinoma (RCC) development in BHD (Birt-Hogg-Dubé) patients has a very high incidence; thus, identifying this rare syndrome at early stages and preventing metastatic spread is crucial. Over the last decade, the advancement of Next Generation Sequencing (NGS) and the implementation of multigene panels for hereditary cancer syndromes (HCS) have led to a subsequent focus on additional genes and variants, including those of uncertain significance (VUS). Here, we describe a novel FLCN variant observed in a subject manifesting disorders that were suspected to be related to BHDS and with a family history of multiple cancers.

[Not a pneumothorax again! Birt-Hogg-Dubé syndrome: a case report]. / "Nicht schon wieder ein Pneumothorax" ­ Fallbericht Birt-Hogg-Dubé-Syndrom.

Case discussion of a 40-year-old male patient with a history of recurrent pneumothoraces due to Birt-Hogg-Dubé syndrome. In addition to conservative treatment of a pneumothorax on the left side, a subtotal parietal pleurectomy on the right side was performed after recurrence of a pneumothorax 6 years later. CT of the thorax showed high-grade structural remodelling of the lung parenchyma with cystic lung lesions on both sides with a diameter of up to 7.5 cm. After exclusion of alpha-1 antitrypsin deficiency, underlying immunological disease, unremarkable family and occupational history, Birt-Hogg-Dubé syndrome was suspected based on the morphological distribution pattern of the cystic lung lesions. Genetic examination helped detect a heterozygous pathogenic variant in the FLCN gene, namely c.1294_1298del;p.(Ser432Argfs*22). Birt-Hogg-Dubé syndrome is a rare genetic disorder clinically characterized by pulmonary cysts, fibrofolliculomas of the skin and occurrence of clustered renal tumors. In particular, the increased risk of renal malignancies and the risk of spontaneous pneumothoraces underlines the importance of early diagnosis and screening of affected patients and their families.

[Pneumothorax with Birt-Hogg-Dubé Syndrome Diagnosed by Family History:Report of a Case].

Birt-Hogg-Dubé (BHD) syndrome is a rare autosomal and predominantly inherited disorder. A 43 year-old woman was admitted to our hospital for right spontaneous pneumothorax and the thoracoscopic pulumonary wedge resection was performed. A chest computed tomography (CT) scan before surgery showed multiple bilatetal thin walled pulmonary cysts predominant to the lower mediastinum side of the lung field. Since her brother had history of pneumothorax with BHD syndrome. Diagnosed by deoxyribonucleic acid (DNA) sequence analysis of his BHD gene, she was diagnosed as BHD syndrome.

[Expert consensus on the diagnosis and management of Birt-Hogg-Dubé syndrome].

Birt-Hogg-Dubé syndrome (BHD) is a rare autosomal dominant disorder characterized by diffuse pulmonary cysts often leading to recurrent spontaneous pneumothorax, cutaneous fibrofolliculomas or trichodiscomas, and a variety of renal cell cancers. It is caused by pathogenic variants in the FLCN gene located on chromosome 17p11.2. Although an increasing number of patients with BHD syndrome are being recognized in China, the missed diagnosis and delayed diagnosis are still common. In addition, appropriate management is difficult for most of them. Pulmonary cysts and pneumothorax are the main presenting features, but skin and renal lesions appear to be less common in Chinese subjects than those reported from European and American countries. Therefore, the consensus is established by experts from the related disciplines to improve the diagnosis and management of BHD syndrome. This consensus consists of 15 recommendations related to BHD syndrome, including clinical assessments, diagnosis, differential diagnosis, treatment, follow-up, and family management. In particular, it provides revised diagnostic criteria based on the Chinese situation. We hope to promote scientific and clinical progress in this rare disease and improve the prognosis of the patients.Summary of recommendations[Recommendation 1]　The folliculin (FLCN) gene is currently the only affirmative causative gene for Birt-Hogg-Dubé (BHD) syndrome, and the pedigree analysis of genetic testing of family members' samples can assist in the rapid identification of causative gene variants. The genetic testing methods, including Sanger sequencing, Multiplex Ligation-dependent Probe Amplification (MLPA), and Next-Generation Sequencing (NGS), can be chosen based on individual patient's care needs. (43 voting experts; 43 in favor, 0 against, 0 abstention).[Recommendation 2]　Patients with BHD syndrome should undergo chest CT scan to evaluate cystic lesions in the lungs, and routine evaluation of the kidneys for tumor foci, using ultrasound, enhanced CT, or MR as appropriate. (43 experts voted; 43 in favor, 0 against, 0 abstention).[Recommendation 3]　Clinicians should establish a diagnosis based on the appropriate clinical presentation and in conjunction with genetic test results and/or a family history of BHD syndrome. (43 voting experts; 43 in favor, 0 against, 0 abstention).[Recommendation 4]　Lung histopathological biopsy is not recommended as the first choice for patients with suspected BHD syndrome based on clinical and pulmonary imaging manifestations. (43 voting experts; 43 in favor, 0 against, 0 abstention).[Recommendation 5]　BHD syndrome should be particularly distinguished from other diffuse cystic lung diseases, such as lymphangioleiomyomatosis (LAM), lymphocyte interstitial pneumonia (LIP), pulmonary Langerhans cell histiocytosis (PLCH), etc. (43 voting experts; 43 in favor, 0 against, 0 abstention).[Recommendation 6]　BHD syndrome is clinically rare and often involves multiple disciplines, such as respiratory and critical care medicine, radiology, pathology, thoracic surgery, urology, genetics, and dermatology, and multidisciplinary discussions are recommended to improve the diagnosis of BHD syndrome. (43 voting experts; 43 in favor, 0 against, 0 abstentions).[Recommendation 7]　Patients with BHD syndrome should avoid smoking, and are recommended to be vaccinated with influenza, pneumococcal, and SARS-Cov-2 vaccines to prevent infections. (43 voting experts; 43 in favor, 0 against, 0 abstentions).[Recommendation 8]　Air travel is not recommended for patients with BHD syndrome who have experienced pneumothorax until it has been recovered. (43 voting experts; 42 in favor, 0 against, 1 abstention).[Recommendation 9]　In patients with BHD syndrome complicated by pneumothorax, early pleurodesis is recommended to reduce the risk of recurrence. (43 voting experts; 43 in favor, 0 against, 0 abstention).[Recommendation 10]　Observation or topical treatment may be chosen for patients with BHD syndrome complicated by fibrofolliculoma or trichodiscoma. (43 voting experts; 42 in favor, 0 against, 1 abstention).[Recommendation 11]　For patients with BHD syndrome complicated by renal tumors, annual abdominal MR examination is recommended when tumors are <1 cm in diameter; when tumors are 1-3 cm in diameter, abdominal MR examination at every 6 months or ablation surgery is recommended; when renal tumors are >3 cm in diameter, local excision of renal tumors with preservation of renal function is recommended. (43 voting experts; 43 in favor, 0 against, 0 abstention).[Recommendation 12]　Patients with BHD syndrome are associated with significantly increased risks of kidney cancer, and routine screenings of kidney cancer during their lifetime are recommended. (43 voting experts; 43 in favor, 0 against, 0 abstention).[Recommendation 13]　Couples with BHD syndrome are advised to undergo prenatal genetic counseling when preparing for pregnancy, to work with a prenatal diagnostician to assess genetic risk, and to discuss the feasibility of prenatal diagnosis during pregnancy. (43 experts voted; 43 in favor, 0 against, 0 abstention).[Recommendation 14]　Family members of patients with BHD syndrome should receive health education, and FLCN gene testing is recommended for asymptomatic adults to rule out BHD syndrome in a timely manner. (43 voting experts; 43 in favor, 0 against, 0 abstention).[Recommendation 15]　Improvements in the clinical diagnosis and treatment of BHD syndrome and overall management are needed. Due to the scarcity of effective therapeutic drugs, multicenter, prospective clinical trials are recommended. (43 experts voted; 43 in favor, 0 against, 0 abstention).

FLCN regulates transferrin receptor 1 transport and iron homeostasis.

Birt-Hogg-Dubé (BHD) syndrome is a multiorgan disorder caused by inactivation of the folliculin (FLCN) protein. Previously, we identified FLCN as a binding protein of Rab11A, a key regulator of the endocytic recycling pathway. This finding implies that the abnormal localization of specific proteins whose transport requires the FLCN-Rab11A complex may contribute to BHD. Here, we used human kidney-derived HEK293 cells as a model, and we report that FLCN promotes the binding of Rab11A with transferrin receptor 1 (TfR1), which is required for iron uptake through continuous trafficking between the cell surface and the cytoplasm. Loss of FLCN attenuated the Rab11A-TfR1 interaction, resulting in delayed recycling transport of TfR1. This delay caused an iron deficiency condition that induced hypoxia-inducible factor (HIF) activity, which was reversed by iron supplementation. In a Drosophila model of BHD syndrome, we further demonstrated that the phenotype of BHD mutant larvae was substantially rescued by an iron-rich diet. These findings reveal a conserved function of FLCN in iron metabolism and may help to elucidate the mechanisms driving BHD syndrome.

Frequency of truncating FLCN variants and Birt-Hogg-Dubé-associated phenotypes in a health care system population.

PURPOSE: Penetrance estimates of Birt-Hogg-Dubé syndrome (BHD)-associated cutaneous, pulmonary, and kidney manifestations are based on clinically ascertained families. In a health care system population, we used a genetics-first approach to estimate the prevalence of pathogenic/likely pathogenic (P/LP) truncating variants in FLCN, which cause BHD, and the penetrance of BHD-related phenotypes. METHODS: Exomes from 135,990 patient-participants in Geisinger's MyCode cohort were assessed for P/LP truncating FLCN variants. BHD-related phenotypes were evaluated from electronic health records. Association between P/LP FLCN variants and BHD-related phenotypes was assessed using Firth's logistic regression. RESULTS: P/LP truncating FLCN variants were identified in 35 individuals (1 in 3234 unrelated individuals), 68.6% of whom had BHD-related phenotype(s), including cystic lung disease (65.7%), pneumothoraces (17.1%), cutaneous manifestations (8.6%), and kidney cancer (2.9%). A total of 4 (11.4%) individuals had prior clinical BHD diagnoses. CONCLUSION: In this health care population, the frequency of P/LP truncating FLCN variants is 60 times higher than the previously reported prevalence. Although most variant-positive individuals had BHD-related phenotypes, a minority were previously clinically diagnosed, likely because cutaneous manifestations, pneumothoraces, and kidney cancer were observed at lower frequencies than in clinical cohorts. Improved clinical recognition of cystic lung disease and education concerning its association with FLCN variants could prompt evaluation for BHD.